While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, DUV and VUV lithography is thought to hold particular promise as the next generation in microfabrication technology. In particular, it is strongly desired that photolithography using an ArF excimer laser as the light source reach the practical level as the micropatterning technique capable of achieving a feature size of 0.13 μm or less and that photolithography using an F2 laser as the light source reach the practical level as the micropatterning technique capable of achieving a feature size of 0.1 μm or less.
In the photolithography using an ArF excimer laser (wavelength 193 nm), as the light source, a high sensitivity resist material capable of achieving a high resolution at a small dose of exposure is needed to prevent the degradation of precise and expensive optical system materials. Among several measures for providing a high sensitivity resist material, and the most common is to select each component which is highly transparent at the wavelength of 193 nm. For example, polyacrylic acid and derivatives thereof, norbornene-maleic anhydride alternating copolymers, polynorbornene and metathesis ring-opening polymers have been proposed as the base resin. This choice is effective in that the transparency of a resin alone is increased. However, the photoacid generator has the problem that increasing its transparency leads to a drop of acid generation efficiency, resulting in a low sensitivity or the lack of thermal stability and storage stability. There is available no photoacid generator which is practically acceptable.
For example, JP-A 7-25846, JP-A 7-28237 and JP-A 8-27102 disclose alkylsulfonium salts which are highly transparent, but unsatisfactory in acid generation efficiency and thermal stability. JP-A 10-319581 discloses alkylarylsulfonium salts which have a high sensitivity and a good balance of transparency and acid generation efficiency, but lack thermal stability and storage stability. Arylsulfonium salts, which are regarded effective in photolithography using a KrF excimer laser, are good in acid generation efficiency, thermal stability and storage stability, but very low transparent to ArF excimer laser light so that the pattern resulting from exposure and development is noticeably tapered. The lack of transparency can be compensated for by thinning the resist film, but such a thin resist film has extremely low etch resistance. This is inadequate as the pattern forming process.
Onium salts are widely used as the photoacid generator. They are neutralized salts between a sulfonium or iodonium cation having an alkyl or aryl group bonded thereto and an anion. Upon exposure to light, the cation is dissociated and the anion is released as an acid. Illustrated in the above-referred patent publications are structures on the cation side of onium salts. It was reported that with respect to resolution and pattern shape, the type of acid generated or anion is closely correlated to the type of acid labile group. Illustratively, regarding those resists based on polyhydroxystyrene and polyhydroxystyrene/(meth)acrylate copolymers, a number of studies using a variety of acids have been reported. For example, U.S. Pat. No. 5,744,537 discloses that a satisfactory pattern shape is obtainable when a photoacid generator capable of generating camphorsulfonic acid is added. However, alicyclic structure-bearing polymers adapted for ArF and F2 laser exposure have a low reactivity for acid elimination. Even when such polymers have the same acid-eliminatable group as the polyhydroxystyrene and polyhydroxystyrene/(meth)acrylate copolymer, elimination reaction does not proceed with camphorsulfonic acid.
On the anion side of onium salts, fluorinated alkyl sulfonic acids having a high acidity are often applied. Illustrative fluorinated alkyl sulfonic acids include trifluoromethanesulfonic acid, nonafluorobutanesulfonic acid, and hexadecafluorooctanesulfonic acid. Also included are fluoro-substituted or fluoroalkyl-substituted arylsulfonic acids. Examples include 4-fluorobenzenesulfonic acid, 3-benzenesulfonic acid, 2-benzenesulfonic acid, 2,4-difluorobenzenesulfonic acid, 2,3-difluorobenzenesulfonic acid, 3,4-difluorobenzenesulfonic acid. 2,6-difluorobenzenesulfonic acid, 3,5-difluorobenzenesulfonic acid, 2,3,4-trifluorobenzenesulfonic acid, 3,4,5-trifluorobenzenesulfonic acid, 2,4,6-trifluorobenzenesulfonic acid, 2,3,4,5,6-pentafluorobenzenesulfonic acid, 4-trifluoromethylbenzenesulfonic acid, 5-trifluoromethylbenzenesulfonic acid, 6-trifluoromethylbenzenesulfonic acid, and 4-trifluoromethylnaphthyl-2-sulfonic acid.
With the advance toward a finer feature size, line edge roughness and a size difference between an isolated pattern and a densely packed pattern, known as I/G bias are regarded problematic. It is well known that even when feature sizes are then same on the mask, a size difference appears between an isolated pattern and a densely packed pattern after development. This problem becomes serious with sizes in excess of the wavelength. This is because a difference in light interference upon image formation between an isolated pattern and a densely packed pattern brings about a difference in optical intensity. For example, FIG. 1 illustrates how the size of line on the ordinate changes when the pitch between repetitive lines of 0.18 microns on the abscissa is changed under optical conditions of wavelength 248 nm, NA 0.6 and σ 0.75. Under the normalization that a line size of 0.18 microns is reached at a 0.36 micron pitch (0.18 micron line+0.18 micron space), the size of an optical image once decrease (or narrows) and then increases (or thickens) as the pitch increases.
Next, the results of determining resist line sizes after development are reported. The resist size and the size of an optical image were determined using simulation software PROLITH2 Ver. 6.0 commercially available from KLA-Tencor Corp. (ex-Finle Co.). The resist size decreases with an increase of pitch and becomes increasingly thinner with the enhancement of acid diffusion. The problem of size dependency on line density that the size of an isolated pattern is thinner than that of a densely packed pattern becomes serious. It is understood from the result of simulation that an effective approach for reducing the line density dependency is to reduce the distance of acid diffusion.
However, if acid diffusion is extremely restrained, the side walls of resist patterns after development are serrated or roughened by standing waves, and line edge roughness is enhanced. The resist cross-sectional configuration of a 0.18 μm line-and-space pattern on a silicon substrate achieved when the distance of acid diffusion is changed is computed using simulation software PROLITH2 Ver. 6.0 by KLA-Tencor Corp., with the results being shown in FIG. 2. It is evident that the serration of side walls by standing waves becomes sharper as the distance of acid diffusion is reduced. For the line edge roughness as observed under overhead SEM, the same tendency is ascertained, that is, line edge roughness increases as acid diffusion decreases. A common approach for reducing the roughness of lines is by increasing the distance of acid diffusion, but this approach fails to improve the line density dependency over a certain limit. For improving line edge roughness, it may be effective to increase the optical contrast. For example, at the same exposure wavelength, the line edge roughness decreases as the line width increases. Even at the same exposure wavelength and line width, the line edge roughness decreases with an increasing NA of a stepper and in the case of repetitive patterns, is smaller with modified illumination (e.g., annular illumination, quadrupole illumination) than with normal illumination and with phase shift masks than with conventional Cr masks. The contrast at pattern line edges is correlated to the line edge roughness so that the line edge roughness is smaller as the line edge contrast is sharper. With respect to exposure wavelength, the line edge roughness is smaller with shorter wavelengths. However, when line edge roughness is compared between KrF exposure and ArF exposure, the ArF exposure is deemed advantageous to optical contrast owing to its shortness of wavelength, but actually, the KrF exposure is advantageous, as reported in SPIE 3999, 264 (2001). This is attributable to the performance difference between KrF and ArF resist materials, indicating that the line edge roughness originating from material factors upon ArF exposure is serious. It would be desirable to have a photoacid generator which improves line edge roughness and at the same time, does not exacerbate line density dependency.